Preheat oven temperature to 400 degrees.Combine flour, sugar, baking powder, and salt in a large bowl. Whisk the dry ingredients together well. Melt butter in a measuring cup that holds about 4 cups of liquid (or a bowl of similar size).
Add 1/2 cup of milk, eggs, and vanilla to the melted butter. Whisk to combine.
Add liquid ingredients to the flour mixture. Stir until no streaks of flour left. Batter should be thick, but if it seems too thick, add a little more milk to the mixture.
Fold in blueberries.
Pour batter into greased 9X13 baking dish. Bake for 25-30 minutes, or until center of cake is done.